Gas‐Phase Anion Photoelectron Spectroscopy of Alkanethiolate‐Protected PtAu₁₂ Superatoms: Charging Energy in Vacuum vs Solution

The charging behavior of molecular Au clusters protected by alkanethiolate (SCnH2n+1=SCn) is, under electrochemical conditions, significantly affected by the penetration of solvents and electrolytes into the SCn layer. In this study, we estimated the charging energy EC(n) associated with [PtAu24(SCn)18]−+e−→[PtAu24(SCn)18]2− (n=4, 8, 12, and 16) in vacuum using mass-selected gas-phase anion photoelectron spectroscopy of [PtAu24(SCn)18]z (z=−1 and −2). The EC(n) values of PtAu24(SCn)18 in vacuum are significantly larger than those in solution and decrease with n in contrast to the behavior reported for Au25(SCn)18 in solution. The effective relative permittivity (ϵm*) of the SCn layer in vacuum is estimated to be 2.3–2.0 based on the double-concentric-capacitor model. Much smaller ϵm* values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of ϵm* with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.